Traditionally, inorganic materials have dominated the electronic device industry. For example, silicon arsenide and gallium arsenide have been used as semiconductor materials, silicon dioxide has been used as an insulator material, and metals such as aluminum and copper have been used as electrode materials. In recent years, however, there has been an increasing research effort aimed at using organic materials rather than the traditional inorganic materials in electronic devices. Among other benefits, the use of organic materials may enable lower cost manufacturing of electronic devices, may enable large area applications, and may enable the use of flexible circuit supports for display backplane and integrated circuits.
Thin-film organic electronics promise lightweight, flexible, inexpensive devices produced using high rate, low cost, solution based methods like spin coating or reel-to-reel processing with compliant substrates (Rogers, J. A., et al., Proc. Nat. Acad. Sci., 2001, 98:4835-4840; Daniel, J. H., et al., ECS Tranactions, 2006, 3:229-236). In a low cost manufacturing environment, process steps like thermal annealing of thin-films to improve charge carrier mobilities (Tunnell, A. J., et al., Org. Electron, 2008, 9:507-514) should occur in air. Thus, it is important that the chosen organic semiconductor possesses both good solubility and excellent stability in air at elevated temperatures.
A variety of organic semiconductor materials have been considered, the most common being fused aromatic ring compounds as exemplified by small molecules such as pentacene-containing compounds, tetracene-containing compounds, anthracene-containing compounds, bis(acenyl)acetylene compounds, and acene-thiophene compounds. Several polymeric materials have also been considered such as regioregular polythiophenes, which are exemplified by poly(3-alkylthiophene), and polymers having fused thiophene units or bis-thiophene units.
Due to the high charge carrier mobilities associated with its thin films, pentacene is one of the most widely utilized organic semiconductor compounds. However, its application in thin-film electronic devices is hindered by its poor solubility and its propensity to photo-oxidize (Ono, K., et al., Tetrahedron, 2007, 61:9699-9704; Palayangoda, S. S., et al., J. Org. Chem., 2007, 72:6584-6587; Etienne, A. and C. Beauvios, Compt. Rend., 1954, 239:64-66; Benor, A., et al., Org. Electron, 2007, 8:749-758; Koch, N., et al., Org. Electron, 2006, 7:537-545). Pentacene oxidation leads to diminished electronic device performance. Pentacene-6,13-dione forms upon photooxidation and has been implicated as a deep charge carrier trap that reduces charge carrier mobility (Koch, N., et al., Org. Electron, 2006, 7:537-545). From a device perspective, larger acenes like hexacene, heptacene, octacene and nonacene are inherently more promising species. They have progressively smaller band gaps and potentially much higher charge carrier mobilities but they are also far more prone to oxidative degradation, a characteristic that has severely limited opportunities in OFET, OLED, OPV and other electronic applications. Thus, hexacene and heptacene have only been isolated in rigid polymeric matrices where they show lifetimes of approximately 12 and 4 hours, respectively. (See Mondal, R.; Adhikari, R. M.; Shah, B. K.; Neckers, D. C. Org. Letters 2007, 9, 2505-2508 and Mondal, R.; Shah, B. K.; Neckers, D. C. J. Amer. Chem. Soc. 2006, 128, 9612-9613.) Octacene and nonacene have never been isolated, nor have any of their derivatives. With its large size and predicted small HOMO-LUMO gap, nonacene in particular is a highly desirable—even if highly challenging—synthetic target. Unsubstituted (i.e., parent) nonacene is predicted to have open shell, singlet diradical, pi-conjugated ground state (Bendikov, M.; Houk, K. N.; Duong, H. M.; Starkey, K.; Carter, E. A.; Wudl, F. J. Amer. Chem. Soc. 2004, 126, 7416-7417) which indicates that even if formed, it would be a highly reactive species with a very short lifetime.
Therefore, what is needed in the art are substituted, soluble nonacene derivatives that possess improved oxidative resistance as compared to unsubstituted nonacene and that can be cast into thin-film organic semiconductor materials for use in electronic applications.